To make sauce, heat oil and butter in large saucepan. Stir in onion, garlic, and pepper flakes, and cook over medium heat until softened, 5 to 7 minutes. Stir in flour and cook 1 to 2 minutes. Whisk in fish broth, chicken broth, water, and salt, and bring to a simmer, stirring. Stir in cream and cook 4 to 5 minutes.

In large bowl mix linguine, sauce, salmon, peas, dill, and lemon juice; pour into casserole dish. Bake 25 to 30 minutes (20 minutes for small casseroles) until brown and bubbling.